A frequency generator is often used in a driving motor of a video tape recorder to drive a rotary cylinder head, in a driving motor of an optical scanning apparatus to drive a rotary multi-plane mirror or in other such driving motors. Such a frequency generator comprises a magnetic flux generator rotatable integrally with a rotor or other rotary member of a motor and a generating coil pattern opposed to the magnetic flux generator.
FIG. 4 shows a motor having such a prior art frequency generator. In FIG. 4, a depressed cup-shaped rotor yoke 32 of a motor 31 includes a rotary shaft 35 in an integral form, and the rotary shaft 35 is rotatably supported by a bearing 40 in a cylindrical holder portion 39 provided in a stationary drum of a rotary cylinder head. A motor mounting portion 38 is provided in a part of the stationary drum in the form of a step. On the motor mounting portion 38 are mounted a printed wiring board 44 and an iron board 46 in a stack, with their center holes engaging the holder portion 39. The iron board 46 is further overlaid with a stator core 42 which encircles the holder portion 39. The stator core 42, iron board 46 and wiring board 44 are fixed on the stationary drum by applying screws passing through themselves to the motor mounting portion 38. A driving coil 43 of a predetermined number of phases is wound on the stator core 42.
On the inner circumferential surface of the rotor yoke 32 is secured a cylindrical rotor magnet 33 whose inner circumferential surface is opposed to the outer circumferential surface of the stator core 42, maintaining a predetermined distance therebetween. The rotor magnet 33 is magnetized in the thickness direction thereof throughout its substantially entire length except its lower end portion as shown in FIG. 6 to form a predetermined number of magnetic poles (6 poles in the illustrated example) in the circumferential direction to behave as a magnet portion 52. The lower end of the rotor magnet 33 is magnetized in the axial direction to form an adequate number of magnetic poles (for example, 24 poles) in the circumferential direction to behave as a magnetic flux generating portion 51 of the frequency generator. On the iron board 46 opposed to the magnetic flux generating portion 51 and via an insulating layer is formed by etching or other process a generating coil pattern 47 of the frequency generator as shown in FIG. 6. The wiring board 44 is a single-surface printed board, and appropriate circuit elements 48 are provided on the printed surface. On the opposite surface of the wiring board 44 is overlaid the iron board 46.
FIG. 5 shows an example where the printed board 44 is a double-surface printed board. On one of the printed surfaces is provided appropriate circuit elements 48, and on the other printed surface is provided the generating coil pattern 47 of the frequency generator, thereby omitting the iron board 46 used in the example of FIG. 4.
According to the prior art arrangement of FIG. 4, the use of both the circuit board and the iron board increases the number of parts and increases the manufacturing cost. The use of the double-surface printed board in the prior art of FIG. 5 also increases the manufacturing cost. Further, in both the prior art examples, the circuit board is manufactured by first etching a large board material to form a number of generating coil patterns thereon and by subsequently press-cutting the large board material into respective circuit boards and concurrently punching out center holes for engagement with the holder portion. Therefore, a positional inconsistency often occurs between the center of the center hole and the center of the generating coil pattern. When the center of the center hole is off the center of the generating coil pattern, the center of the generating coil pattern deviates from the rotation center when the center hole of the circuit board engages the holder portion. This increases the wow flutter of the output of the frequency generator.